Add multi-monitor support

[Plenvorik]

Problem

The app only considers the primary monitor's boundaries. Multi-monitor setups
are not correctly supported.

Current code

src/main.cpp:450-460

int screen_width = GetSystemMetrics(SM_CXSCREEN);
int screen_height = GetSystemMetrics(SM_CYSCREEN);

Solution

Use EnumDisplayMonitors() or GetMonitorInfo() to correctly handle all monitors.

Reference

See IMPROVEMENTS.md for implementation example.